US8331802B2 - Synchronous circuit for use in optical homodyne receiver for generating local oscillation light with accurate demodulation - Google Patents

Synchronous circuit for use in optical homodyne receiver for generating local oscillation light with accurate demodulation Download PDF

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US8331802B2
US8331802B2 US12/801,972 US80197210A US8331802B2 US 8331802 B2 US8331802 B2 US 8331802B2 US 80197210 A US80197210 A US 80197210A US 8331802 B2 US8331802 B2 US 8331802B2
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signal
optical
phase
local oscillation
oscillation light
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US20110008061A1 (en
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Akihiro Fujii
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Oki Electric Industry Co Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/63—Homodyne, i.e. coherent receivers where the local oscillator is locked in frequency and phase to the carrier signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers

Definitions

  • the present invention relates to an optical homodyne receiver, and more particularly a synchronous circuit for use in an optical homodyne receiver adapted to receive an optical signal modulated, for example, by BPSK (Binary Phase-Shift Keying).
  • BPSK Binary Phase-Shift Keying
  • phase modulation such as NRZ (Non-Return-to-Zero)-BPSK, which is more advantageous in signal-to-noise ratio than the conventional intensity modulation.
  • the phase modulation utilizes the coherence of light so as to render its phase convey information to be transmitted to thereby forward the information.
  • receiver schemes such as heterodyne and homodyne detections have predominantly been proposed.
  • a receiver end prepares a carrier wave accurately synchronous in phase with a modulated signal received as input signal, and utilizes the interference therebetween to thereby demodulate the signal.
  • a beat signal caused by interference between a local oscillation light and a carrier wave slightly different in frequency from each other is detected to thereby determine the phase state of a received signal necessary for demodulation.
  • the local oscillation light is generated by a local oscillation light source.
  • a receiver end With the homodyne detection, a receiver end generates a carrier wave accurately conformed in both frequency and phase to a received signal, and utilizes the interference therebetween to thereby determine the phase state of the received signal necessary for demodulation.
  • These schemes can be implemented by means of the phase locking of a received input signal light to the local oscillation light.
  • the heterodyne detection scheme does not require an accurate phase locking of the received input signal light with the local oscillation light, thus being considered to be higher in implementation.
  • heterodyne detection is lower in reception sensitivity than the homodyne detection by about 3 dB.
  • the BPSK signal when using such an optical phase-locked loop to demodulate, for example, a BPSK signal with a modulation index of 100%, the BPSK signal in itself does not include a spectral component of a carrier wave. It is therefore necessary to use some measure for extracting a phase difference of the carrier wave from the local oscillation light.
  • measures such as a multiply method and a Costas loop have been used for many years.
  • a carrier wave is modulated in phase in response to binary values so as to be shifted by an angle of ⁇ (radian).
  • ⁇ radian
  • the start phases 0 and ⁇ corresponding to the binary values of the carrier in a modulated signal are doubled to cause the phase differences 2 ⁇ between the respective time slots of the binary values. Therefore, the periodicity of trigonometric functions causes the binary values in the frequency-doubled signal to have the same phase. As a result, a stable signal having a frequency equal to the doubled carrier frequency can be extracted by the multiply method.
  • a circuit configuration where a Costas loop is applied to optical communications is disclosed in, for example, Y. Chiou, et al., “Effect of Optical Amplifier Noise on Laser Linewidth Requirements in Long Haul Optical Fiber Communication Systems with Costas PLL Receivers”, Journal of Lightwave Technology, Vol. 14, No. 10, pp. 2126-2134 (1996).
  • a carrier frequency uses a band enormously lower in frequency than in optical communications. Therefore, the above-described solutions are effective. However, in the optical communications using a carrier frequency of several hundred THz, it is difficult to use these solutions without modification.
  • the BPSK signal having information conveyed on a carrier of several hundred THz needs to be multiplied “literally”.
  • multiplication is difficult to implement by means of currently existing electronic devices due to physical characteristic requirements of circuit components involved in the configuration.
  • a nonlinear optical effect can be utilized to generate harmonics.
  • the generation of harmonics involves many research issues remain unsolved such as difficulties about a wavelength region and conversion efficiency.
  • phase modulation effect such as nonlinear optical effect
  • phase information of a received signal light in itself may be caused to change. Therefore, this solution is difficult to reduce to practice.
  • a bottleneck in circuit configuration is a multiplier multiplying an I-axis and a Q-axis signal.
  • This multiplier is required to accurately multiply output signals having components of an I-axis and a Q-axis having phases shifted by ⁇ /2. This requirement is caused by the fact that a result from the multiplying corresponds to a phase difference.
  • multipliers which can accurately multiply signals in a low frequency bandwidth, and multipliers operable in a relatively high frequency bandwidth of several ten GHz.
  • this multiplier is also required to have its input bandwidth of which the upper limit is correspondingly high to the baud rate of the high frequency signal dealt with. That is caused by the above-described cos ( ⁇ +d) and sin ( ⁇ +d) including a baseband signal component d.
  • the baseband is so broad that the highest frequency included in the baseband component exceeds the upper limit of the multiplier operable.
  • the I-axis and Q-axis signal components have the high frequency components, exceeding the operable band, cut off by a low-pass filter in the multiplier to thereby cause the signals per se to be smoothed.
  • the multiplier develops on its output a result from multiplying these smoothed signals with each other, which does not accurately reflect the phase error. Therefore, with the optical communications, it is difficult to maintain a stable operation.
  • asynchronous circuit for use in an optical homodyne receiver for synchronizing a received local oscillation light with a received optical BPSK (Binary Phase-Shift Keying) signal includes: a synchronizing signal generator for receiving the optical BPSK signal and the local oscillation light, combining either of the received optical BPSK signal and the received local oscillation light with a phase-shifted signal to generate a plurality of different optical signals, at least one of which for use in producing a signal demodulated from the optical BPSK signal is square-law detected, calculating the square-law detected optical signal to convert a resultant optical signal into a corresponding electric signal, and producing an electric phase-locking signal which will be a demodulated signal from the optical BPSK signal on the basis of the converted electric signal; an intensity-modulating circuit for using the phase-locking signal as a modulating signal to intensity-modulate an incident continuous light into an optical intensity-modulated signal; an optoelectric converting circuit for optoelectrically converting the
  • a synchronizing signal generator combines either of a received optical BPSK signal and a received local oscillation light with a phase-shifted signal to generate a plurality of different optical signals, at least one of which for use in generating a signal demodulated from the optical BPSK signal is square-law detected, calculates the square-law detected optical signals to convert the obtained optical signal into an electric signal, and produces an electric phase-locking signal which will be a demodulated signal from the optical BPSK signal on the basis of the converted electric signal.
  • the phase-locking signal is used as a modulating signal by an intensity-modulating circuit to modulate an incident continuous light into an optical intensity-modulated signal, which is optoelectrically converted and square-law detected by an optoelectric converting circuit.
  • the square-law detected signal is used by an optical VCO circuit as a phase error signal to adjust a phase or frequency of the local oscillation light to be generated.
  • the local oscillation light with its phase or frequency thus adjusted is supplied to the synchronizing signal generator.
  • the phase error signal can thus be obtained without using an electric multiplier. As a result, even for a signal modulated with an enormously high carrier frequency to which an electric multiplier cannot be applied, the local oscillation light in accurate synchronization can be generated with the accuracy of demodulation improved.
  • the present invention does not require, or at least can minimize, elements or portions which need accurate adjustment.
  • FIG. 1 is a schematic block diagram showing the configuration of an illustrative embodiment of a synchronous circuit of an optical homodyne receiver to which the present invention is applied;
  • FIG. 2 is a schematic block diagram showing the configuration of an illustrative embodiment of an optical VCO in the synchronous circuit of the optical homodyne receiver shown in FIG. 1 ;
  • FIG. 3 is a schematic block diagram showing the configuration of a substantial part of an alternative embodiment of the synchronous circuit of the optical homodyne receiver to which the present invention is applied;
  • FIG. 4 is a schematic block diagram showing the configuration of a substantial part of a still alternative embodiment of the synchronous circuit of the optical homodyne receiver to which the present invention is applied;
  • FIG. 5 is a schematic block diagram showing the configuration of a substantial part including a power controller additionally provided to the configuration shown in FIGS. 3 and 4 ;
  • FIG. 6 is a schematic block diagram showing the configuration of an illustrative embodiment of an optical homodyne receiver in accordance with the present invention.
  • a synchronous circuit 10 for use in an optical homodyne receiver in accordance with a preferred embodiment generally includes: a synchronizing signal generator 10 A adapted to combine a received optical BPSK signal 40 with a received local oscillation light 44 , either of which the generator 10 A may shift in phase, to thereby produce a plurality of different optical signals, among which optical signals 48 and 52 for use in producing a signal demodulated from the optical BPSK signal are square-law detected, optical signals resultant from the square-law detection being operated and converted to an electric signal 56 , from which a phase-locking electric signal 58 is produced which may be used as a signal demodulated from the optical BPSK signal; an intensity-modulating circuit 10 B adapted to use the phase-locking signal 58 as a modulation signal to modulate an incident continuous light 62 into
  • the phase error signal can be obtained.
  • the local oscillation light accurately synchronous can be generated to thereby render the accuracy of demodulation improved.
  • both circuit arms of the circuitry are required to be adjusted in skew.
  • the present invention does not require, or at least can minimize, elements or portions which require accurate adjustment.
  • the synchronous circuit 10 for use in an optical homodyne receiver, e.g. 140 , FIG. 6 , in accordance with the preferred embodiment is adapted to receive an optical BPSK signal 38 as an object to be demodulated, and demodulate the received optical BPSK signal to thereby obtain information the transmitter end intended to forward.
  • the synchronous circuit 10 of the optical receiver shown in FIG. 1 mainly has a synchronization function of rendering a carrier component of the BPSK signal to be demodulated in phase with the local oscillation light, and to output an optical demodulated signal.
  • the synchronous circuit 10 of the optical homodyne receiver generally includes, as shown in FIG. 1 , the synchronizing signal generator 10 A, the intensity-modulating circuit 10 B, the optoelectric converting circuit 10 C, and the optical VCO (Optical Voltage-Controlled Oscillator) circuit 10 D, which are interconnected as depicted.
  • the synchronizing signal generator 10 A the intensity-modulating circuit 10 B
  • the optoelectric converting circuit 10 C the optoelectric converting circuit
  • optical VCO Optical Voltage-Controlled Oscillator
  • the synchronizing signal generator 10 A has the functions of receiving an optical BPSK signal 40 from a polarization controller 12 and a local oscillation light 44 to shift in phase the received optical BPSK signal 40 and the local oscillation light 44 , and combining the received or shifted optical BPSK signal with the shifted or unshifted local oscillation light to thereby produce a plurality of different optical signals, one of which for use in producing a demodulated signal from the optical BPSK signal is square-law detected, a difference being calculated between the square-law detected optical signals to be converted in the form of optical signal to a corresponding electric signal, from which a phase-locking electric signal is generated which will possibly be used to demodulate the optical BPSK signal.
  • the synchronizing signal generator 10 A includes a 3-dB coupler 14 , a balanced photo detector 16 , and a driver amplifier 18 which are interconnected as shown.
  • the intensity-modulating circuit 10 B has a function of using the phase-locking signal as a modulating signal to modulate an incident continuous light 58 into an optical intensity-modulated signal 66 .
  • the intensity-modulating circuit 10 B includes a driver 20 for modulator, a CW (Continuous Wave) light source 22 , an intensity modulator 24 , and an optical amplifier 26 , which are interconnected as shown.
  • the optoelectric converting circuit 100 has a function to optoelectrically convert the optical intensity-modulated signal 66 to square-law detect the converted signal.
  • the optoelectric converting circuit 10 C includes an optoelectric converter 28 , a loop filter 30 , and a bias adder 32 interconnected as illustrated.
  • the optical VCO circuit 10 D has a function to use the square-law detected signal 72 as a phase error signal to generate a local oscillation light 46 with its phase or frequency changed or adjusted accordingly.
  • the optical VCO circuit 10 D includes an optical amplifier 36 and an optical VCO 34 that are interconnected as shown.
  • the polarization controller 12 has a function to receive a BPSK signal 38 to be demodulated to rotate the polarization plane of the received BPSK signal 38 to be demodulated so as to be consistent with the polarization plane of a local oscillation light generated by a local oscillation light source included in the optical VCO 34 as described later.
  • the polarization controller 12 outputs the BPSK signal 40 to be demodulated with its polarization plane thus conformed to the polarization plane of the local oscillation light to one terminal 42 of the 3-dB coupler 14 .
  • the 3-dB coupler 14 generally has a function to combine the received BPSK signal 40 to be demodulated with the local oscillation light 44 to output two resultant lights 48 and 52 thus combined.
  • the local oscillation light 46 is supplied from the other terminal 44 .
  • the 3-dB coupler 14 gives a phase shift of ⁇ /2 to the local oscillation light 46 , and combines the BPSK signal 40 to be demodulated with the local oscillation light thus phase-shifted to thereby produce a first combined light 48 from its one output terminal S 1 to a non-inverting (+) terminal 50 of the balanced photo detector 16 .
  • the 3-dB coupler 14 gives the phase shift of ⁇ /2 to the BPSK signal 40 to be demodulated, and combines the BPSK signal thus phase-shifted with the local oscillation light 46 , not phase-shifted, to thereby produce a second combined light 52 from the other output terminal S 2 to an inverting ( ⁇ ) terminal 54 of the photo detector 16 .
  • the balanced photo detector 16 has a function to output the electric signal 56 , which is obtained by subtracting a signal resultant from squaring the second combined light 52 from another signal resultant from squaring the first combined light 48 .
  • the balanced photo detector 16 outputs as the demodulated signal 56 a signal having its amplitude depending on information conveyed on the BPSK signal 40 to be demodulated.
  • the information is represented by a phase ⁇ /2 or ⁇ /2, or a code “0” or “1”.
  • the balanced photo detector 16 outputs such a signal as the demodulated signal 56 to a subsequent code-identifying circuit, not shown, and the driver amplifier 18 .
  • the demodulated signal 56 is compared with a threshold value for code discrimination by the code-identifying circuit at the midway timing of an eye pattern to thereby be decoded into demodulated data for utilization.
  • the driver amplifier 18 may be an RF (Radio Frequency) amplifier, which is adapted to amplify components of the baseband of a signal received.
  • the driver amplifier 18 feeds the driver 20 for modulator as a feedback signal with a signal 58 resultant from amplifying the baseband of the supplied demodulated signal 56 .
  • the driver 20 for modulator has a function to change a bias of the feedback signal 58 received from the driver amplifier 18 , and to produce a drive signal 60 for the intensity modulator 24 .
  • the bias is changed in order to enable the intensity modulator 24 to properly intensity-modulate the continuous light 62 incident from the CW light source 22 .
  • the bias value is optimized so as to linearly perform the intensity modulation.
  • a Mach-Zehnder type of intensity modulator is commercially available which has its DC input port and its DC bias voltage variable.
  • the driver 20 for modulator may not be required.
  • the driver 20 for modulator supplies the produced drive signal 60 to the intensity modulator 24 .
  • the CW light source 22 has a function to generate the continuous light 62 having a predetermined wavelength to allow the generated continuous light 62 to be incident to the intensity modulator 24 .
  • the continuous light 62 generated by the CW light source 22 functions as a carrier for use in the intensity modulator 24 . Therefore, the continuous light 62 is set sufficiently higher in frequency than the drive signal 60 inputted to the intensity modulator 24 .
  • the continuous light 62 generated by the CW light source 22 is not synchronized at all, namely, asynchronous with the received BPSK signal 38 to be demodulated and the received local oscillation light 46 .
  • the intensity modulator 24 is adapted to modulate the intensity of the continuous light 62 from the CW light source 22 with the amplitude of the drive signal 60 from the driver 20 for modulator.
  • a Mach-Zehnder intensity modulator can be applied to the intensity modulator 24 .
  • the intensity modulator 24 outputs the optical intensity-modulated signal 64 to the optical amplifier 26 .
  • the optical amplifier 26 is adapted for amplifying the optical intensity-modulated signal 64 outputted from the intensity modulator 24 .
  • the optical amplifier 26 amplifies the received optical intensity-modulated signal 64 , and outputs the amplified optical intensity-modulated signal 66 to the optoelectric converter 28 of the optoelectric converting circuit 10 C.
  • the optoelectric converter 28 has a function to optoelectrically convert the amplified optical intensity-modulated signal 66 .
  • the optoelectric converter 28 square-law detects the amplified optical intensity-modulated signal 66 .
  • the optoelectric converter 28 outputs an output signal 68 substantially corresponding to the square of an output signal from the driver amplifier 18 .
  • the square-law detection renders the output signal 68 from the optoelectric converter 28 free from the elements of information contained in the BPSK signal to be demodulated.
  • the optoelectric converter 28 sends the output signal 68 to the loop filter 30 .
  • the loop filter 30 has a low pass characteristic to remove a high frequency component from the output signal 68 provided from the optoelectric converter 28 to output a phase error signal 70 .
  • the low pass characteristic of the loop filter 30 defines a response rate of a phase-locked loop in this embodiment.
  • the loop filter 30 outputs the resultant phase error signal 70 to the bias adder 32 .
  • the bias adder 32 has a function to change the bias value of the phase error signal 70 including a DC component to cause an electric VCO 80 , FIG. 2 , included in the optical VCO 34 to change or adjust its oscillation frequency range accordingly.
  • the bias adder 32 outputs a phase error signal 72 having its bias value changed to the optical VCO 34 of the optical VCO circuit 10 D.
  • the optical VCO 34 is responsive to the phase error signal 72 having its bias value changed to generate a local oscillation light 74 with the phase thereof controlled accordingly.
  • the optical VCO 34 includes, as shown in FIG. 2 , an electric VCO 80 , an RF hybrid 81 , a CW light source 82 , phase shifters 83 a and 83 b , driver amplifiers 84 a and 84 b , and SSB (Single SideBand) modulator 85 , which are interconnected as depicted.
  • the optical VCO 34 in this embodiment is thus different from the optical VCO disclosed in Camatel, et al., described earlier.
  • the electric VCO 80 is operative in response to the phase error signal 72 outputted in the form of voltage signal by the bias adder 32 and having its bias value changed to accordingly control, e.g. change, the oscillation frequency by itself.
  • the electric VCO 80 outputs to the RF hybrid 81 an oscillation signal 86 depending on the phase error signal 72 .
  • the RF hybrid 81 functions as providing the output signal 86 from the electric VCO 80 with a phase difference of ⁇ /2 therebetween to thereby produce two components 81 a and 81 b.
  • the RF hybrid 81 delivers the signals 81 a and 81 b having the phase difference from each other to the phase shifters 83 a and 83 b , respectively.
  • the phase shifters 83 a and 83 b have the phase adjustment function of providing the optical SSB modulator 85 with the signals 83 c and 83 d with the phase difference maintained.
  • the phase shifters 83 a and 83 b output the signals 83 c and 83 d having the phase thereof adjusted to the driver amplifiers 84 a and 84 b , respectively.
  • the driver amplifiers 84 a and 84 b function as amplifying the output signals from the RF hybrid 81 to a level required for enabling the optical SSB modulator 85 to be operable.
  • the driver amplifiers 84 a and 84 b amplify the input signals 83 c and 83 d to such a level to deliver the resultant signals 84 c and 84 d thus amplified to the input ports 85 a and 85 b of the optical SSB modulator 85 , respectively.
  • the CW light source 82 is adapted to generate a continuous light 88 having its frequency substantially equal to a carrier frequency of the BPSK signal 38 to be demodulated.
  • the CW light source 82 emits the generated continuous light 88 to the optical SSB modulator 85 .
  • the optical SSB modulator 85 is an optical modulator adapted to respond to the oscillation signal 86 from the electric VCO 80 to phase-modulate the continuous light 88 provided from the CW light source 82 to generate a local oscillation light 74 which has reflected the phase error. More specifically, the optical SSB modulator 85 is able to shift the frequency of the continuous light 88 from the CW light source 82 by an amount corresponding to the frequency of the RF signal from the electric VCO 80 . For instance, where the BPSK signal has its central frequency f [Hz] and the CW light source 82 has its central frequencies f ⁇ f [Hz] and ⁇ f [Hz], the optical SSB modulator 85 produces its output having its frequency shifted by ⁇ f to be f [Hz].
  • the optical SSB modulator 85 could likewise be a phase modulator adapted to output the spectra having frequency components shifted or a Mach-Zehnder type of modulator.
  • phase modulator would cause the output spectra to be frequency modulated spectrum components such that plural spectra appear at the central frequency of f ⁇ f at the intervals of ⁇ f on the lower and higher sides of the central frequency. That may cause the baseband of the BPSK signal to have a region common to the train of spectra other than the frequency of f, sometimes causing crosstalk. In some cases, therefore, it is required to provide an optical bandpass filter immediately after the output of the phase modulator to thereby extract the frequency f only.
  • the baseband of the BPSK and the frequency of f ⁇ 2 ⁇ f may include a region common to each other, sometimes causing a crosstalk. In such a case also, it may therefore be required to provide an optical bandpass filter immediately after the output of the Mach-Zehnder modulator to thereby extract the frequency f only.
  • the optical SSB modulator 85 which is free from the situations stated above and capable of generating a local oscillation light from the output of the CW light source 82 over the entire range of optical intensity except for the loss that could be caused by the junction portions. From the optical SSB modulator 85 , the optical VCO 34 outputs the local oscillation light 74 with its phase thus controlled to the optical amplifier 36 shown in FIG. 1 .
  • the optical amplifier 36 has a function to amplify the local oscillation light 74 having reflected the phase error.
  • the optical amplifier 36 inputs the amplified local oscillation light 46 to the other input terminal 44 of the 3-dB coupler 14 .
  • the optical amplifier 36 preferably has a variable amplification function, as will be described later.
  • the optical amplifier 36 may not be included when the local oscillation light is sufficient in intensity.
  • the synchronous circuit 10 in the optical receiver receives a BPSK signal 38 to be demodulated by the polarization controller 12 .
  • the received BPSK signal 38 to be demodulated will have its polarization plane conformed to the polarization plane of the local oscillation light to be outputted as the BPSK signal 40 to be demodulated.
  • the BPSK signal 40 to be demodulated is inputted to the one terminal 42 of the 3-dB coupler 14 .
  • the local oscillation light 74 outputted from the optical VCO 34 is amplified through the optical amplifier 36 , and inputted to the other terminal 44 of the 3-dB coupler 14 as the local oscillation light 46 .
  • the BPSK signal 40 to be demodulated is combined with the local oscillation light 46 through the 3-dB coupler 14 .
  • the 3-dB coupler 14 phase-shifts the local oscillation light 46 by an angle of ⁇ /2, and combines the light of BPSK signal 40 to be demodulated with the local oscillation light thus shifted in phase to output the resultant first combined light 48 from its one output terminal S 1 to the balanced photo detector 16 .
  • the 3-dB coupler 14 also phase-shifts the BPSK signal 40 to be demodulated by an angle of ⁇ /2, and combines the local oscillation light 46 with the light of optical signal thus phase-shifted to output the resultant second combined light 52 from its other output terminal S 2 to the balanced photo detector 16 .
  • the balanced photo detector 16 subtracts the signal obtained by squaring the inputted second combined light 52 from the signal obtained by squaring the inputted first combined light 48 to output the resultant electric signal 56 .
  • Phase parameters ⁇ S and ⁇ LO are phase differences representing a fluctuation caused by the instability of a light source or the like.
  • Parameters E S and E LO represent respective amplitude components.
  • a phase parameter d is an element of information carried on the BPSK signal 40 to be demodulated, that is, a stream of data taking the values thereof equal to ⁇ /2 or ⁇ /2 with respect to 0 rad, which is employed in the illustrative embodiment.
  • the first and second combined lights S 1 ( 48 ) and S 2 ( 52 ) outputted from the 3-dB coupler 14 can respectively be represented by expressions (3) and (4):
  • an optical signal e 1 ( ⁇ /2) represents an optical signal obtained by giving a phase shift of ⁇ /2 to the optical signal e 1
  • an optical signal e 2 ( ⁇ /2) represents an optical signal obtained by giving a phase shift of ⁇ /2 to the optical signal e 2 .
  • the electric signal E OUT ( 56 ) outputted from the balanced photo detector 16 is an amplitude-modulated signal depending on an element of information carried on the BPSK signal 40 to be demodulated while a phase-locked state is established.
  • This amplitude-modulated signal is supplied as a temporarily demodulated signal to, for example, a subsequent code-identifying circuit, not shown, which will extract a stream of codes the transmitter end intended to forward.
  • the stream of codes will be demodulated data for utilization.
  • the electric signal E OUT ( 56 ) outputted from the balanced photo detector 16 has its DC component cut off and the components of the remaining bandwidth amplified, and is supplied to the driver 20 for modulator.
  • the driver 20 for modulator changes the bias of an inputted signal to supply the drive signal 60 to the intensity modulator 24 .
  • the intensity modulator 24 is driven with the drive signal 60 . More specifically, the intensity modulator 24 modulates the intensity of the continuous light 62 from the CW light source 22 depending on the amplitude of the drive signal 60 from the driver 20 for modulator.
  • the optical intensity-modulated signal 64 outputted from the intensity modulator 24 is amplified by the optical amplifier 26 .
  • the optoelectric converter 28 optoelectrically converts the amplified optical intensity-modulated signal 66 supplied from the intensity modulator 24 , while it square-law detects the optical intensity-modulated signal 66 .
  • a square-law detected signal E O/E in the form of electric signal is represented by an expression (6):
  • the element d of information as a phase parameter is in the form of data stream taking the value thereof equal to ⁇ /2 or ⁇ /2. Therefore, a phase parameter 2d is equal to ⁇ or ⁇ . According to the nature of a cosine function in trigonometric algebra, the symbol 2d has only its sign inverted.
  • the square-law detected signal E O/E is obtained which does not include the element d of information conveyed on the BPSK signal to be demodulated.
  • This square-law detected signal E O/E represents a difference in angular frequency between the BPSK signal to be demodulated and the local oscillation light. Particularly, assuming that the carrier frequencies are equal, this signal E O/E represents a difference in phase.
  • the first term is a DC component representing a constant value while the second term is an AC component reflecting the phase error.
  • the AC component of the second term is passed through the loop filter 30 having a low pass characteristic to thereby be integrated, thus being rectified to a DC level or smoothed.
  • the component of the first term is passed through the loop filter 30 without being changed.
  • the bias adder 32 cancels the first term by addition and subtraction of DC voltages.
  • phase error signal E VCO ( 72 ) inputted to the optical VCO 34 can be represented by an expression (7):
  • the optical VCO 34 generates the local oscillation light 74 having the phase thereof controlled depending on the phase error signal E VCO ( 72 ) having its bias value changed.
  • the local oscillation light 74 is inputted as the local oscillation light 46 through the optical amplifier 36 to the input terminal 44 of the 3-dB coupler 14 .
  • phase-locked loop causes the synchronous circuit 10 in the optical receiver to synchronize, or lock, the phases of the BPSK signal to be demodulated and local oscillation light with each other.
  • the phase error is generally controlled so as to be equal to zero.
  • the phase error is controlled so as to be equal to ⁇ /4.
  • the phase error signal 70 as a feedback signal is generated by square-law detection by intensity-modulating the continuous light 62 and optoelectrically converting the optical intensity-modulated signal 64 .
  • the phase error signal 70 can be obtained.
  • the local oscillation light accurately synchronous can be generated to thereby improve the accuracy of demodulation.
  • the length or other property of wiring in a circuit outputting components of an I-axis and a Q-axis has to be adjusted.
  • the illustrative embodiment does not need multiplication on two streams of signal, so that elements or portions which require accurate adjustment are not involved, or at least can be minimized.
  • Alternative embodiments of the synchronous circuit 10 for use in an optical receiver may include a 90-degree hybrid coupler 90 , a balanced photo detector 92 and a driver amplifier 94 , in addition to the polarization controller 12 , the balanced photo detector 16 , the driver amplifier 18 , the driver 20 for modulator, the CW light source 22 , the intensity modulator 24 , the optical amplifiers 26 and 36 , the optoelectric converter 28 , the loop filter 30 , the bias adder 32 and the optical VCO 34 .
  • FIG. 3 shows an alternative embodiment of the synchronizing signal generator 10 A serving as a substantial portion of the synchronous circuit 10 in the optical receiver.
  • the synchronizing signal generator 10 A is so partially depicted in the figure in order to clearly understand differences only in terms of components, elements and interconnections different from those shown in and described with reference to FIG. 1 .
  • FIG. 3 therefore shows the synchronizing signal generator 10 A including the 90-degree hybrid coupler 90 , the balanced photo detectors 16 and 92 , and the driver amplifiers 18 and 94 , which are interconnected as illustrated.
  • the connection relationship in an optical receiver of the synchronous circuit 10 including the generator 10 A shown in FIG. 3 may be the same as FIG. 1 of Chiou, et al., described earlier.
  • the 90-degree hybrid coupler 90 receives a BPSK signal 40 to be demodulated on its one terminal 96 .
  • the 90-degree hybrid coupler 90 also receives the local oscillation light 46 on its other terminal 98 .
  • the 90-degree hybrid coupler 90 includes first and second beam combiners, and a 90-degree phase shifter, not shown.
  • the first beam combiner in the 90-degree hybrid coupler 90 combines the BPSK signal 40 to be demodulated with the local oscillation light to thereby obtain an aggregate component and a difference component between the BPSK signal to be demodulated and the local oscillation light.
  • the 90-degree hybrid coupler 90 supplies the obtained aggregate and difference components 104 and 106 from its output terminals 100 and 102 to inverting ( ⁇ ) and non-inverting (+) terminals of the balanced photo detector 92 , respectively.
  • the second beam combiner also not shown, combines the BPSK signal 40 to be demodulated with an optical signal obtained by phase-shifting the local oscillation light by ⁇ /2 or 90° to thereby obtain an aggregate component and a difference component between the BPSK signal to be demodulated and the optical signal thus phase-shifted by 90°.
  • the 90-degree hybrid coupler 90 supplies the obtained aggregate and difference components 112 and 114 from output terminals 108 and 110 to non-inverting (+) and inverting ( ⁇ ) terminals of the balanced photo detector 16 , respectively.
  • the balanced photo detector 92 has a function to produce a demodulated signal on the basis of the supplied aggregate and difference components 104 and 106 .
  • the balanced photo detector 92 includes a couple of photo detectors, not shown.
  • the supplied aggregate and difference components 104 and 106 are optoelectrically converted each.
  • the balanced photo detector 92 then subtracts an optoelectrically converted signal of the difference component 106 from an optoelectrically converted signal of the aggregate component 104 to output a signal thus obtained as a demodulated signal 116 to the driver amplifier 94 .
  • the balanced photo detector 16 also has a function to produce a feedback signal for synchronization on the basis of the supplied aggregate and difference components 112 and 114 .
  • the balanced photo detector 16 includes a couple of photo detectors, also not shown.
  • the supplied aggregate and difference components 112 and 114 are optoelectrically converted each.
  • the balanced photo detector 92 then subtracts an optoelectrically converted signal of the aggregate component 112 from an optoelectrically converted signal of the difference component 114 to output a signal thus obtained as a feedback signal 118 to the driver amplifier 18 .
  • a demodulated signal E DEMOD ( 122 ) and a feedback signal EFB ( 120 ) supplied to the driver 20 for modulator, respectively can be represented by expressions (11) and (12):
  • E DEMODI 1 2 ⁇ E S ⁇ E LO ⁇ cos ⁇ ( ⁇ S - ⁇ LO + d ) ( 11 )
  • the feedback signal 56 inputted to the driver 20 for modulator has the same phase component. Therefore, a phase locking process may follow similarly that of the previous embodiment.
  • the feedback signal 118 from the balanced photo detector 16 is amplified by the driver amplifier 18 to be outputted as the demodulated signal 120 to a subsequent stage.
  • the demodulated signal ( 116 ) is amplified and outputted by the driver amplifier 94 .
  • the feedback signal ( 118 ) is used as a demodulated signal for use in the phase-locked loop.
  • the phase error signal can be obtained without using an electric multiplier.
  • the synchronous circuit 10 in the optical receiver not adapted to multiple outputs from symmetrical circuits, removes, or at least can minimize, elements or portions which need the accurate adjustment.
  • a demodulated signal can be obtained from a portion not related to the phase-locked loop.
  • the synchronous circuit 10 for use in an optical homodyne optical receiver in accordance with a further alternative embodiment of the present invention is also directed to an optical BPSK signal.
  • Like components and elements are also designated with the same reference numerals and repetitive description thereon will be omitted.
  • the synchronous circuit 10 including the synchronizing signal generator 10 A shown in FIG. 4 may be the same as FIG. 1 , as may be the case with FIG. 3 , except for the components and elements shown in FIG. 4 .
  • the polarization controller 12 and the components and elements subsequent to the driver 20 for modulator may be common between both embodiments.
  • FIG. 4 shows a substantial portion of the synchronous circuit 10 in an optical receiver including an optoelectric converter 124 in addition to the 90-degree hybrid coupler 90 , the balanced photo detector 16 , and the driver amplifiers 94 and 18 .
  • the 90-degree hybrid coupler 90 includes a first beam combiner, not shown, adapted to combine a BPSK signal 40 to be demodulated with the local oscillation light 46 to thereby obtain an aggregate component and a difference component between the BPSK signal 40 to be demodulated and the local oscillation light 46 .
  • the 90-degree hybrid coupler 90 also includes a second beam combiner, not shown, adapted to combine optical signals obtained by respectively phase-shifting the BPSK signal 40 to be demodulated and the local oscillation light 46 by ⁇ /2 to thereby obtain an aggregate component and a difference component between the optical signals thus shifted in phase.
  • the 90-degree hybrid coupler 90 outputs the aggregate component 104 from the first beam combiner on its port a ( 100 ) as an optical demodulated signal 104 .
  • the 90-degree hybrid coupler 90 also outputs the difference component 106 from the first beam combiner on its port b ( 102 ) to the non-inverting (+) terminal of the balanced photo detector 16 .
  • the 90-degree hybrid coupler 90 outputs the aggregate component 112 from the second beam combiner on its port c ( 108 ) to the optoelectric converter 124 .
  • the 90-degree hybrid coupler 90 then outputs the difference component 114 from the second beam combiner on its port d ( 110 ) to the inverting ( ⁇ ) terminal of the balanced photo detector 16 .
  • the optoelectric converter 124 has a function to optoelectrically convert an optical signal 112 outputted from the port c ( 108 ) of the 90-degree hybrid coupler 90 , and square-law detect the resultant electric signal.
  • the optoelectric converter 124 outputs the square-law detected signal obtained through the optoelectric conversion to the driver amplifier 94 as a feedback signal 126 to the phase-locked loop.
  • the driver amplifier 94 amplifies the supplied signal 126 , which will be delivered to the driver 20 for modulator as an amplified feedback signal 128 .
  • the balanced photo detector 16 has a function to produce a demodulated signal 130 on the basis of the supplied optical signals 106 and 114 . More specifically, the balanced photo detector 16 , on the one hand, optoelectrically converts and square-law detects the optical signal 106 outputted from the port ( 102 ) b of the 90-degree hybrid coupler 90 , and, on the other hand, optoelectically converts and square-law detects the optical signal 114 outputted from the port d ( 110 ) of the 90-degree hybrid coupler 90 . The photo detector 16 in turn subtracts the latter square-law detected signal from the former square-law detected signal to output a resultant signal as the demodulated signal 130 to the driver amplifier 18 . The driver amplifier 18 amplifies the supplied demodulated signal 130 , and outputs an amplified demodulated signal 132 to a subsequent stage in the optical receiver.
  • the optical signal 112 outputted from the port c ( 108 ) of the 90-degree hybrid coupler 90 is outputted as an optical demodulated signal to the optoelectric converter 124 .
  • the optical demodulated signal 112 is a PSK/OOK (Phase-Shift Keying/On-Off Keying) optical signal obtained through optical modulation.
  • the BPSK signal e 1 to be demodulated and the local oscillation light, e 2 may be represented by the expressions (1) and (2), respectively.
  • Signals E a to E d obtained by square-law detecting the optical signals 104 , 106 , 112 , and 114 , respectively, on the ports a to d of the 90-degree hybrid coupler 90 can be represented by expressions (13) through (16):
  • E a 1 8 ⁇ ⁇ E S + E LO - 2 ⁇ E S ⁇ E LO ⁇ cos ⁇ ( ⁇ S - ⁇ LO + d ) ⁇ ( 13 )
  • E b 1 8 ⁇ ⁇ E S + E LO + 2 ⁇ E S ⁇ E LO ⁇ cos ⁇ ( ⁇ S - ⁇ LO + d ) ⁇ ( 14 )
  • E c 1 8 ⁇ ⁇ E S + E LO + 2 ⁇ E S ⁇ E LO ⁇ sin ⁇ ( ⁇ S - ⁇ LO + d ) ⁇ ( 15 )
  • E d 1 8 ⁇ ⁇ E S + E LO - 2 ⁇ E S ⁇ E LO ⁇ sin ⁇ ( ⁇ S - ⁇ LO + d ) ⁇ ( 16 )
  • the optical demodulated signal E c represented by the expression (15) is the output signal 126 from the optoelectric converter 124 , as described above.
  • the optical demodulated signal E a includes a governing term influenced by angular frequencies ⁇ S and ⁇ LO , and the element d of information conveyed on the BPSK signal 40 to be demodulated and the other terms.
  • the feedback signal E c may be treated similarly to the signal delivered to the phase-locked loop in the expression (5) in the first embodiment or the feedback signal 120 delivered to the phase-locked loop in the expression (11) in the alternative embodiment described earlier. Therefore, when inputting the feedback signal E c ( 126 ) to the driver amplifier 94 , the phase error signal E VCO ( 72 ) outputted from the bias adder 32 can also be represented by an expression (17):
  • the balanced photo detector 16 subtracts the square-law detected signal of the optical signal 114 outputted from the d port ( 110 ) of the 90-degree hybrid coupler 90 from the square-law detected signal of the optical signal 106 outputted from the b port ( 102 ) of the 90-degree hybrid coupler 90 to output the resultant signal 130 .
  • This output signal 130 is amplified through the driver amplifier 18 to be developed as the demodulated signal 132 .
  • the optical signal as a subtrahend of the subtraction is the optical signal E b in the expression (14), and the optical signal as a minuend of the subtraction is the optical signal E d in the expression (16). Therefore, the output signal E OUT ( 130 ) from the balanced photo detector 16 can be represented by expressions (19-1) to (19-3):
  • the expressions (19-2) and (19-3) are obtained by modifying the expression (19-1), as will be described below. Specifically, a value of ⁇ /2 is applied to a cosine term in the expression (19-1) to change the cosine term to a sine term, and thereafter a formula is applied which transforms a sum of two sine terms to a product of a cosine term and a sine term. Since the cosine part in the product of the cosine term and the sine term does not include any variable, it can be replaced by a constant to thereby obtain the expression (19-2). Furthermore, to the expression (19-2) the parameter ⁇ defined in the expression (18) is applied, thereby obtaining the expression (19-3).
  • the demodulated signal E OUT ( 130 ) outputted from the balanced photo detector 16 includes a component of sin ( ⁇ +d).
  • the phase error signal E VCO ( 112 ) includes a component of sin (2 ⁇ ) for use in demodulation for a conventional Costas loop. Therefore, the operation of the phase-locked loop in this embodiment, including the demodulation, may be completely the same as a Costas loop.
  • the balanced photo detector 16 in this alternative embodiment is particularly featured as completely cancelling out a phase offset of ⁇ /4 reflected in the phase error signal in the first embodiment by means of a phase offset of ⁇ /4 of the demodulated signal produced by balance-detecting the output signals 106 and 114 from the ports b ( 102 ) and d ( 110 ).
  • the output signal 104 from the port a ( 100 ) of the 90-degree hybrid coupler 90 remains as an optical signal in the form of OOK signal converted from the BPSK signal, without being converted to an electric signal. This is also one feature of the present alternative embodiment.
  • the synchronous circuit 10 is preferably provided with a power controller 134 , as shown in FIG. 5 .
  • the optical amplifier 36 in the optical VCO circuit 10 D may be implemented by an amplifier having its gain controllable.
  • the power controller 134 may preferably be applied to the configuration of the embodiments shown in FIGS. 3 and 4 .
  • the power controller 134 has a function to receive the BPSK signal 40 to be demodulated and the local oscillation light 44 to monitor the power of the received optical signals, and to adjust or control the gain of the optical amplifier 36 having amplification function so as to substantially equalize the power of both signals depending on a result from the monitoring.
  • the power controller 134 receives the BPSK signal 40 to be demodulated branched from the interconnection to the 90-degree hybrid coupler 90 in order to monitor its power, and also receives the local oscillation light 74 branched from the interconnection 44 from the optical VCO 34 in order to monitor its power.
  • the power controller 134 generates a control signal controlling the gain of the optical amplifier 36 such as to substantially equalize the monitored power of both signals, and supplies the generated control signal 136 to the optical amplifier 36 having amplification function.
  • the optical amplifier 36 having amplification function adjusts the power level of the output signal 44 in response to the supplied control signal 136 to output the adjusted output signal 44 to the 90-degree hybrid coupler 90 .
  • the phase error signal is generated by square-law detection by intensity-modulating the continuous light by a signal similar to the obtained demodulated signal, and optoelectrically converting the obtained optical intensity-modulated signal. Therefore, without using an electric multiplier, the phase error signal can be obtained.
  • the synchronous circuit 10 as a result, even for a signal modulated with an enormously high carrier frequency such as several hundred THz to which an electric multiplier cannot be applied, the local oscillation light in accurate synchronization can be generated to thereby improve the accuracy of demodulation.
  • the instant alternative embodiment also can remove, or at least minimize, elements or portions which need accurate adjustment.
  • this alternative embodiment has such an advantageous effect that the relationship between the demodulated signal 130 for the inputted BPSK signal 40 to be demodulated and the feedback signal 126 for phase locking can be similar to that of a conventional solution. Additionally, from the inputted BPSK signal 40 to be demodulated, the electric signal 130 and the optical signal 112 can be obtained, and when the BPSK signal to be demodulated and the local oscillation light have the same power, the optical demodulated signal is a BPSK/OOK signal resultant from optical modulation, which is also advantageous.
  • the above-described embodiments are intended to demodulate a signal BPSK-modulated with a too much high carrier frequency such as several hundred THz.
  • the present invention is not to be restricted to these specific embodiments, but may also be applicable to demodulating a BPSK signal lower in carrier frequency than the above-described embodiments.
  • the intensity modulator 24 is adapted to intensity-modulate the continuous light from the CW light source 22 to obtain an optical intensity-modulated signal.
  • a semiconductor laser may be applicable such as to modulate a drive current for pumping with a high frequency signal or a pulse signal to thereby directly modulate the intensity of an oscillation light.
  • the synchronous circuit 10 in the optical receiver may be adapted to utilize this feature of the semiconductor laser to obtain an optical intensity-modulated signal depending on a demodulated signal or a signal different only in phase from such a demodulated signal.
  • the alternative embodiment described above is adapted to input the component of the I-axis to the phase-locked loop, and extract the component of the Q-axis as a demodulated signal.
  • the component of the I-axis may be extracted as a demodulated signal.
  • the component of the I-axis may be inputted to the phase-locked loop, and extracted as a demodulated signal.
  • the component of the Q-axis may be inputted to the phase-locked loop, and extracted as a demodulated signal.
  • an optical homodyne receiver 140 includes the polarization controller 12 , the synchronous circuit 10 , and a code-identifying circuit 142 .
  • the code-identifying circuit 142 has a function to compare a signal supplied as a demodulated signal 56 , also shown in FIG. 1 , with a threshold value for code discrimination at a midway timing in an eye pattern of this signal to produce demodulated data.
  • the code-identifying circuit 142 receives the demodulated signal 56 , and compares the received demodulated signal 56 with the threshold value for code discrimination to produce demodulated data 144 to a utility circuit, not shown.

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  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
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